For example, in a color image forming apparatus that includes process cartridges of four colors cyan, magenta, yellow, and black, a belt conveyor apparatus (one example of an “image carrier”) is positioned in order to convey a sheet to each process cartridge to perform image transfer. In this image forming apparatus, since the aforementioned belt conveyor apparatus is detachably mountable with respect to a main unit of the image forming apparatus for the purpose of improving the workability of sheet jam processing or maintenance, it is necessary to detect whether or not the belt conveyor apparatus is in the appropriate position.
A technique (hereunder, referred to as a “conventional technique”) already exists in which a reflection-type photosensor is positioned as image density detecting means, and when a value of a received light amount at a light receiving portion of the photosensor deviates from a region that is used in image density detection, it is determined that there is no belt conveyor apparatus.
However, in the above described conventional technique, a configuration is adopted in which a light emitting portion of the photosensor emits light at a constant level, and the presence or absence of a belt conveyor apparatus is detected in accordance with whether or not the received light amount level of a light receiving portion at that time is outside a predetermined region. Consequently, for example, a received light amount level at the light receiving portion may decrease due to degradation of elements in the light receiving portion or light emitting portion of the aforementioned photosensor, which can result in a mistaken determination that there is no belt conveyor apparatus even though there is a belt conveyor apparatus.
For example, when degradation of elements of a light emitting portion or a light receiving portion occurs, because the difference in the level of a detected light amount of the light receiving portion will become smaller between a time when an image carrier is present and when there is no image carrier in accordance with the degradation, the determination accuracy with respect to whether or not the image carrier is disposed at a predetermined position will decline.